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Postretirement benefits (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Net periodic benefit income        
Interest cost $ 242 $ 478 $ 484 $ 956
Expected return on plan assets (1,875) (1,905) (3,748) (3,809)
Amortization of prior service credit (355) (355) (711) (711)
Amortization of net actuarial losses 407 575 814 1,150
Net periodic benefit income $ (1,581) $ (1,207) $ (3,161) $ (2,414)